Nitriding of iron-based alloys in a gaseous ammonia atmosphere at elevated temperatures has been practiced for many years to produce hard, wear-resistant surfaces on steel parts. The ammonia dissociates, or decomposes, to release atomic nitrogen, [N], which reacts with alloying elements (e.g., aluminum, chromium, vanadium, etc.) which have been added to the steel to improve nitriding response, by forming finely dispersed nitride particles which impart the hard layer to the surface of the metal parts. Since nitrides from this group of alloying elements are somewhat unstable, tending to coarsen at temperatures in excess of about 1200.degree. F., (which results in softening of the surface), conventional nitriding is carried out at temperatures of about 1000.degree. F. The resulting nitrided parts are then limited to maximum service temperatures significantly below 1000.degree. F. Further, because of the relatively low treatment temperatures, the diffusion of nitrogen is slow, and nitriding treatment times of up to 50 hours are often needed to achieve hardened surface layers in the range of 0.010 to 0.020 inches thickness. In the case of stainless steels nitrided for improved surface hardness, corrosion resistance is normally reduced because the major element, chromium, is precipitated from the base material as a nitride and is no longer free to perform its role as the solid solution element which makes the alloy "stainless".
Recently, titanium-alloyed steels have been nitrided. It has been demonstrated that titanium nitride particles are very stable in a steel matrix, even at temperatures in the vicinity of 2000.degree. F. Thin-section iron-titanium alloy parts have been nitrided throughout their cross section to produce very high strength alloys. Similarly, through nitriding has been done with titanium-containing austenitic stainless steels as disclosed in Kindlimann U.S. Pat. No. 3,804,678, entitled "Stainless Steel by Internal Nitridation". The teachings of this prior patent, might, at first glance, appear applicable to other classes of stainless steels, i.e. ferritic stainless steels, however, on further analysis, the internal nitridation of the normally non-hardenable ferritic grades of stainless steels is not indicated. Chen, for example, found embrittlement due to the massive chromium nitrides formed when he attempted to nitride iron alloys containing 26 percent chromium and 3 and 5 percent titanium (F. P. H. Chen, "Dispersion Strengthening of Iron Alloys by Internal Nitriding", PhD Thesis, Rensselaer Polytechnic Institute, Troy N.Y. (August 1965)). Similarly, it was found that when titanium containing austenitic stainless steels are subjected to nitridation in such a manner as to achieve the low interparticle spacing of stable nitride particles as claimed in U.S. Pat. No. 3,804,678, massive chromium nitrides are also formed during the treatment. While such chromium nitrides may be eliminated by a denitriding step involving a treatment in a nitrogen-free atmosphere at elevated temperatures after the nitridation step, such removal tends to leave relatively large subsurface pores in the stainless steel surface. These pores lead to reduced tensile strength and ductility, and lower creep strength. See L. E. Kindlimann and G. S. Ansell, "Dispersion Strengthening Austenitic Stainless Steel by Nitriding", Metallurgical Transactions, Vol. 1 (February, 1970) pp 507-515. Furthermore, in this article, the authors observed that such pore formation became more severe at lower nitriding temperatures, i.e. below about 1900.degree. F.
While subsurface pores may be eliminated through a post-nitriding hot working step used to bond packets of thin strip or powder into heavier gage sheet, bars, forms, etc., this consolidation step is costly, particularly when the final gage sheet required is within the capability of the through nitriding process. Elimination of the pores, while maximizing high temperature strength of nitride strengthened ferritic stainless steel grades in thin gages up to about 0.020 inches thickness, has important engineering and economic implications to design and fabrication of energy saving heat recovery devices. For such an application, ferritic stainless steels are preferred over austenitic types because of lower thermal expansion (lower thermal stress and less distortion), higher resistance to oxide scaling (longer life and/or lighter weight), and freedom from stress corrosion cracking (catastrophic failure). Nonetheless, implementation of standard ferritic grades has been hampered by low strength at elevated temperatures, and the use of more costly higher strength nickel based and cobalt based alloys is often found necessary. The result is a long pay-back period for the energy recovery devices, which has adversely influenced acceptance of the need to install heat recovery devices.